OCDM-based all optical multi-level security

ABSTRACT

A high data rate optical signal is inverse multiplexed into a multitude of lower-rate tributaries, each of which is coded by its unique OCDM code, and the combined coded tributaries are injected into a common phase scrambler. Coherent summation of these optically encoded tributaries pass through a shared phase or phase and frequency scrambler before exiting the secure location. The setting of the scrambler acts as the key. The authorized recipient with the correct key retrieves the ones and zeros of the several decoded signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.Provisional Application No. 61/098,418, filed on Sep. 19, 2008, thedisclosures of which are incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Funding for research was made with Government support underMDA972-O3-C-0078 awarded by the Defense Advanced Research ProjectsAgency (DARPA). The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to optical code divisionmultiplexing (OCDM) systems and methodologies using passive integratedphase coders for multi-level security (MLS) in existing wavelengthdivision multiplexing (WDM) networks.

2. Description of the Related Art

Faced with the demand for high capacity communication, there is growinginterest in deploying WDM fiber optic networks. For many applications,for example in avionics, an optical physical layer that can supportmulti-level security is needed. Using different fibers (space divisionmultiplexing) is an obvious choice, but is costly because it requiresadditional fiber infrastructure. In wavelength division multiplexingnetworks, different security levels are carried on the same fiber, buton different optical channels. However, concerns remain regardingwavelength division multiplexing enabled multi-level security beingsusceptible to eavesdropping through inter-window cross talk as well asthrough more conventional means of eavesdropping.

Conventional optical eavesdropping can include, for example, physicaltapping of the fiber.

An example of eavesdropping via crosstalk is described below.

In a dense wavelength division multiplex (DWDM) system, each of severalinput signals enter a DWDM node or network element and is assigned orconverted to a specific wavelength, typically, in the 1550 nanometer(nm) band. After wavelength conversion, each individual signalwavelength or channel is then multiplexed by wavelength divisionmultiplexing and transmitted onto the same fiber. Consequently, a singlefiber carries more than one wavelength. In fact each wavelength carriedby a DWDM system may be considered a virtual fiber.

The signal carried on the virtual fibers of DWDM systems may besusceptible to eavesdropping of a form that is not possible if thesignals are on separate fibers. In DWDM systems different channelstravel through the same fiber and the same components. As a result ofcross-talk, nonlinearity, etc., at the receiving end, there may be aresidual of signal(s) from other channels that can be isolated,amplified and detected.

The potential for eavesdropping may be better appreciated by referenceto FIG. 1 where there is depicted a receiving node 100 in a DWDMnetwork. Receiving node 100 may be an optical demultiplexer or add dropmultiplexer, a wavelength converter, or an optical cross-connect thatserves as a drop off or interchange point for one or more channels. FIG.2 shows, on a logarithmic scale, the optical spectrum of channel 10 inFIG. 1 as it dropped from node 100. As FIG. 2 shows, although the goalwas to drop only channel 10, channel 11 is clearly visible. In FIG. 3, afilter is used to reduce the optical signal to noise ratio (OSNR) forchannel 10. As FIG. 3 shows, channel 11 is still present with enoughpower to be recoverable. In fact, in FIG. 4, the channel 10 transmitterhas been turned off and as FIG. 4 shows there is a significant amount ofresidual power still present from channel 11. Results similar to thoseshown in FIG. 4 have also been achieved by introducing a second filterto attenuate channel 10 in the received spectrum. In either case, inFIG. 4, channel 11 is leaked with large enough OSNR to be recoverableafter optical amplification. Accordingly, the user of channel 10 may beable to recover channel 11 without the network operator ever knowing ofthe breach in security. On another level, residual power from eachchannel may be available on all the channels thereby providing forsecurity akin to having a party line.

BRIEF SUMMARY OF THE INVENTION

An aspect of the present invention is an optical system for transportingdata. The optical system has a source for generating a sequence ofoptical pulses each optical pulse comprising a plurality of spectrallines uniformly spaced in frequency with fixed absolute frequency andrelative phase; a set of data modulators each associated with atributary/channel and operable to modulate the sequence of pulses usingtributary/channel data to produce a modulated data signal; a set ofencoders imparting a plurality of orthogonal codes, each of theplurality of orthogonal codes being associated with one of the datamodulators to spectrally encode the modulated data signal to produce anencoded data signal; an optical combiner for bit-synchronously combiningthe encoded data signals into a composite transport data signal whereineach tributary/channel shares the same spectral bandwidth duringtransmission; a shared spectral phase scrambler for phase scrambling thecomposite transport data signal using a shared scramble code as anencryption key to generate the encrypted signal; a shared spectral phasedescrambler for descrambling the encrypted signal using a shareddescramble code as a key to generate a descrambled data signal, theshared descramble code being a compliment of the shared scramble code;and a set of decoders each of which is a conjugate match to one of theencoders, for spectrally decoding a specified encoded data signal toproduce a set of decoded data signals.

Another aspect of the present invention is a method for transmittingdata from a plurality of tributaries between a transmitting end and areceiving end. The method includes inverse multiplexing a high data rateoptical signal into a plurality of lower-rate tributaries; encoding eachof the plurality of lower-rate tributaries into a plurality of codedtributaries by applying a unique OCDM code from a set of mutuallyorthogonal codes; bit-synchronously combining the plurality of codedtributaries; and phase scrambling the combined coded tributaries using ashared scrambling code as an encryption key to generate an encryptedcomposite signal.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects and advantages of the presentinvention will become better understood with regard to the followingdescriptions, appended claims, and accompanying drawings wherein:

FIG. 1 illustratively depicts a receiving node in a DWDM network.

FIG. 2 is a spectral plot of a received channel that is dropped from theDWDM node of FIG. 1, channel 11 appears as cross-talk in the spectralplot of FIG. 2.

FIG. 3 is a spectral plot of the spectrum in FIG. 2 after filtering.

FIG. 4 depicts the leaked channel after channel 10 in FIG. 3 is turnedoff or after a second filter is applied to FIG. 3.

FIG. 5 illustrates the SPE-OCDM system architecture of an embodiment ofthe present invention. Top X-Y plots show the time-frequency response ateach position identified by large vertical arrows.

FIG. 6 illustrates a fourth-order micro-ring resonator filter.

FIG. 7 is a schematic of the optical circuit, incorporating fourth-ordermicro-ring resonator filters.

FIG. 8 shows a sketch of compatibility of an OCDM system with existingWDM networks. The second and fifth window from the right carryindependent groups of OCDM signals.

FIG. 9 illustrates the effect of scrambling on four Hadamard-32 signals.Each panel is the simulated temporal intensity variation for two bitperiods as might be seen by an eavesdropper.

FIG. 10 shows the progression of increased security by doing frequencyshuffling in addition to shared phase scrambling by using monomialmatrix and the methodology to implement it. Specifically, FIG. 10( a) isthe phase scrambling diagonal matrix that is applied aggregate of allchannels/codes present as a separate unit 540; in FIG. 10( b) the matrixshows combined functionality of phase scrambling in 549 and frequencyshuffling that has to be applied at the position of each code 530present before aggregating, and (c) is the monomially modified Hadamardcodes that represents the phases of each frequency bin.

FIG. 11 illustrates optical time gating for multi-channel interference(MCI) rejection.

DETAILED DESCRIPTION OF THE INVENTION List of Acronyms

The acronyms in the following list are applied at various locationsherein. The meaning of the terms referenced by these acronyms is morecompletely understood from the complete description.

CDMA—code division multiple access

DWDM—dense wavelength division multiplexing

MLL—mode-locked laser

MLS—multi-level security

MCI—multi-channel interference

MRR—micro-ring resonators

OCDM—optical code division multiplex/ing

OCDMA—optical code-division multiple access

OOK—on-off-keying

OSNR—optical signal to noise ratio

PLS—photonic layer security

SPE—spectral phase encoding

WDM—wavelength division multiplexing

High Level Description

New techniques, including an optical code-division multiplexing (OCDM)system and methodology based on a passive spectral phase encoding (SPE)scheme that is compatible with WDM networks and offers photonic layersecurity (PLS) are presented.

Compatibility is achieved through the ability to access and modifyoptical phase of tightly spaced phase locked laser lines with highresolution. In WDM networks, different security levels are carriedthrough different optical windows. However, as discussed above, WDMenabled MLS is susceptible to eavesdropping through inter-window crosstalk as well as to more conventional tapping.

The inventive techniques offer another level of security beyond WDM byproviding the proper recipient its unique OCDM code, without whichinadvertently leaked or intentionally captured signals cannot bedeciphered.

The inventive techniques also offer a higher level of security that isrobust to both exhaustive search and archival attacks through phasescrambling of the inverse multiplexed tributaries of the high data rateaggregate signal.

System Architecture

FIG. 5 illustratively depicts an OCDM system 500 in accordance with anaspect of the present invention. In addition to illustrating an overallsystem architecture, FIG. 5 also includes a diagram that depicts signalflows through the system 500 in the time and frequency domain.

The system 500 comprises a laser source 510 that generates a sequence ofoptical pulses that are fed to each of data modulators 520 _(1 . . . N).The system 500 includes N tributaries or channels that each provide datathat is used to respectively modulate the sequence of optical pulsesgenerated by laser source 510. Each of data modulators 520 _(1 . . . N)comprises an ON/OFF keyed data modulator wherein a “1” symbol or bit inthe digital data stream corresponds to the presence of an optical pulseand a “0” symbol or bit corresponds to the absence of an optical pulse.In accordance with the present invention, other modulation techniquesincluding those based on phase can be applied. In this way, each pulserepresents a bit of information. For example, in a modulated digitaldata stream comprising a “1010” data sequence, each time slot with thebit “1” will result in the presence of an optical pulse whereas eachtime slot with a “0” bit indicates the absence of an optical pulse.

Each modulated data stream is then fed to a corresponding one ofspectral phase encoders 530 _(1 . . . N). As is discussed in furtherdetail below, each of the spectral phase encoders 530 _(1 . . . N) usesa phase mask to apply a phase code associated with a tributary orchannel to each optical pulse in the data stream to produce an encodeddata stream. The phase code operates to provide a “lock” so that only acorresponding phase decoder with the appropriate “key” or phaseconjugate of the phase code of the spectral phase encoder may unlock theencoded data stream. Typically, a spectral phase encoder is associatedwith a particular tributary or channel and therefore allows only anothertributary or channel with the appropriate key to decode or receiveinformation from the particular tributary or channel The informationappears as noise to tributaries or channels that do not have theappropriate key.

After a modulated data stream is encoded, the encoded data stream can bepassively combined with other encoded data streams with bit-timesynchronization, each of which have their own unique spectral phasecodes but overlap completely in the frequency domain. This form ofpassive multiplexing distinguishes optical CDMA from dense wavelengthdivision multiplexing (DWDM) systems where tributaries or channels areassigned independent, non-overlapping spectral passbands.

The combined encoded data streams then pass through a phase scrambler540 which changes the phase of the aggregate signal within eachfrequency bin relative to other frequency bins using a random key sharedwith the receiving end. The scrambled data stream may then betransported over a network, such as a WDM network, to the receiving endwhere a descrambler 550, using the shared random key, undoes thescrambling.

Scrambling the phase of aggregate signals can also be achieved bycombining the spectral phase encoder 530 and the phase scrambler 540 ina single unit. This results in the use of one less coder/scrambler unit.But the more important use of this procedure enables frequency shufflingin addition to aggregate phase scrambling using monomial matrix, asillustrated in FIG. 10, instead of a diagonal matrix.

The descrambled data stream is then fed to a spectral phase decoder 560that, preferably, includes a phase mask that applies the phase conjugateof the phase code of the spectral phase encoder 530, as discussed above.The spectral phase decoder 530 provides a decoded data stream to anoptical time gate 570. As is discussed in detail below, the optical timegate 570 operates to reduce multiple access interference by temporallyextracting only a desired tributary from among the decoded stream. Theoptical time gate 570 produces a data stream, which is fed to a datademodulator 580. Where ON/OFF keying was employed at the transmittingend, the data demodulator 580 comprises an amplitude detector thatreproduces the digital data stream.

Implementation of the above described system is discussed in furtherdetail below.

Implementation of SPE-OCDM

U.S. patent application Ser. No. 11/062,090 describes awavelength-division multiplexing-compatible spectral phase encodingapproach to OCDM, the contents of which are incorporated by reference inthe present application.

In accordance with an aspect of the present invention, the laser source510 comprising a mode locked laser (MLL) having a spectral contentcomprising a stable comb of closely spaced phase-locked frequencies. Thefrequency or comb spacing is deter mined by the pulse repetition rate ofthe MLL.

The laser source 510 may, for example, comprise a ring laser that may beformed using a semiconductor optical amplifier or erbium doped fiberamplifier. The ring laser includes, for example, a laser cavity, amodulator, a wavelength division multiplexer, and a tap point forproviding an output signal, which comprises optical pulses.

Referring to FIG. 5, the MLL produces as its output a stream of shortoptical pulses 512 in the time domain. The pulsed signal can also beshown to be equivalent to a comb of phase-locked continuous wave opticalfrequencies 514 equally spaced on a frequency grid determined by thelaser repetition rate.

As an example, the present invention uses 8 or 16 equally spacedphase-locked laser lines confined to an 80 GHz window depending on thedata rate for individual channels. This 80 GHz window is considered tocomprise 8 or 16 frequency bins, each bin being phase encoded using acoder, to be described below, based on an ultrahigh resolutiondemultiplexer.

In comparison to prior art SPE that use the continuous broad spectrum ofan ultrashort pulse source, the technique disclosed in the presentinvention has the advantage of confining the data-modulated MLL lines totheir respective phase coded frequency bins and all frequency bins to asmall tunable window. The narrower spectral extent of the coded signalalso limits the impact of the transmission impairments such asdispersion and makes this system compatible with standard WDM opticalnetworks.

The output signal 512 is provided to each of data modulators 520 _(1-N).N tributaries or channels in the system provides data 522 _(1-N) that isused to respectively modulate the pulse train or output signal 512. Datamodulators 520 _(1-N) operate to provide ON/OFF keying resulting intime-domain signal 524. In time domain signal 524, the pulses with asolid outline indicates a “1” symbol or bit and pulses with a faintoutline represents a “0” symbol or bit, as previously discussed. Thespectral content of such a signal is shown in frequency plot 526 in FIG.5.

Each of the modulated optical pulse signals are then fed to respectivespectral phase encoders 530 _(1-N). Encoding consists of separating eachfrequency bin, shifting its phase, in this case by 0 or π, as prescribedby the choice of code, and recombining the frequency bins to produce thecoded signal. When the relative phases of the frequencies are shifted,the set of frequencies is unaltered, but their recombination results ina different temporal pattern, e.g., a pulse shifted to a different partof the bit period, multiple pulses within the bit period, or noise-likedistribution of optical power. Each OCDM code is desirably defined by aunique choice of phase shifts.

In accordance with the present invention, a set of Hadamard codes, whichare orthogonal and binary, can be chosen as a coding scheme. This choiceis desirable in that it can achieve relatively high spectral efficiencywith minimal multi-channel interference (MCI). Specifically, this codingscheme offers orthogonality in the sense the MCI is zero at the timethat the decoded signal is maximum. The number of orthogonal codes isequal to the number of frequency bins; hence, relatively high spectralefficiency is possible. Binary Hadamard codes are converted to phasecodes by assigning to +1's and −1's phase shifts of 0 and π,respectively. To encode data, which contains a spread of frequencies, asopposed to the unmodulated pulse stream, which contains only the initialcomb of frequencies produced by the MLL, it is preferable to definefrequency bins around the center of frequencies. Encoding data thenconsists of applying the phase shift associated with a frequency to theentire bin. The output of the phase encoder is then a signal obtained bysumming the phase-shifted frequency components of the modulated signal.

Applying any of these orthogonal codes (except for the case of Code 1,which leaves all phases unchanged) results in a temporal pattern whichhas zero optical power at the instant in time where the initial pulsewould have had its maximum power. Although this choice of orthogonalcodes implies synchronicity as a system requirement, sincedesynchronization will move unwanted optical power into the desiredsignal's time slot, careful code selection allows some relaxation ofthis requirement. For example, simulations indicate that for fourtributaries transmitting at 2.5 Gb/s and using a suitably chosen set offour codes among a set of 15 Hadamard codes of length 16, up to 15 ps ofrelative delay can be tolerated with a power penalty within 1 dB at aBER of 10⁻⁹. Better resiliency to asynchronism may be achieved by usingmultiphase codes.

Phase coding of the individual spectral components requires ademultiplexer with sufficient resolution and path length stability and ameans of shifting phases independently for each frequency. In anembodiment of the present invention, ring-resonator-based photonicintegrated circuits are used to perform coding/decoding functions.

The use of ring-resonator-based circuits in an OCDMA system based onspectral-phase encoding of phase-locked lines of a MLL has beendemonstrated by Anjali Agarwal et al. in “Fully ProgrammableRing-Resonator-Based Integrated Photonic Circuit for Phase CoherentApplications,” IEEE J. of Lightwave Technology, Vol. 24, No. 1, January2006, pp. 77-87, the contents of which are incorporated by reference inthe present application.

Below, the construction and characterization of ring-resonator-basedphotonic integrated circuits and how they can be used to performcoding/decoding functions are described.

As illustrated in FIG. 6, a fourth-order micro-ring resonator filter 600is the basic building block for a coder/decoder. It comprises fourmicro-rings 601 that are vertically coupled to a pair of input 602 andoutput 603 bus waveguides. Vertical coupling allows for more precisecontrol of the coupling strength than lateral coupling, since thevertical separation of the guides depends on the thickness of theintervening layer and is not determined by mask error, photolithography,or etching, all of which are more difficult to control with the requiredprecision and reproducibility. In high-order micro-ring resonator (MRR)filters that are designed to have a maximally flat passband, thecoupling between the bus waveguide 602, 603 and the ring 601 needs to bestrong, whereas the coupling between adjacent rings 601 is designed tobe weak. In order to achieve strong coupling between ring and bus inlateral configuration, the gap between the two would be sub-resolution,and therefore, subject to large random deviations. Vertical couplingallows the ring 601 and bus 602, 603 to come into close proximitywithout the need to etch an ultranarrow coupling gap. Instead thecoupling is determined by well-controlled material deposition. The rings601 support resonant travelling wave modes and the resonant condition isdetermined by the circumference and effective index of the rings 601. Atresonant wavelengths, optical power can be transferred completely fromone bus waveguide 602 to the other 603 via the rings 601, as shown by λ1in FIG. 6, while off-resonant wavelengths λ2, λ3, . . . bypass the rings601. The shape and bandwidth of the filter response is determined by thenumber of rings in the filter, the mutual coupling strength among therings, and between the outer rings and the bus waveguides. Byappropriately coupling multiple rings, the frequency response of thefilters can be tailored to a desired response. As the number of coupledrings increase, the order of the filter increases, leading to a box-likefilter response.

FIG. 7 illustrates an exemplary coder/decoder circuit 700. The codercircuit 700 consists of a common input bus 701 and a common output bus702, with fourth-order micro-ring resonators serving aswavelength-selective cross connects between the two. A fourth-orderfilter cell 703 occupies an on-chip area of 100×400 μm, allowing a largenumber of filter cells on a chip (64 filter cells on a 17×17 mm chip).Each filter 703 is independently tunable in wavelength and each passbandrepresents a frequency bin. An independent heater is placed over each ofthe four rings and can be differentially adjusted to fine tune theoptical line shape.

The relative phase shift between two adjacent frequency bins iscontrolled by a separate thermo-optic phase heater 704, shown in hatchmarks in FIG. 7, and can be continuously varied between 0 and π. Hence,the micro-rings 703 provide the wavelength selectivity, and thethereto-optic heater 704 is used to control the relative phase ofindividual wavelengths.

Due to the symmetry of the above-described configuration, the opticalpath lengths from the input to the output are the same for allwavelengths, and hence, the original phase relationships are maintainedfor all wavelengths when the phase heaters are not activated.

The three necessary functions, frequency demultiplexing, phase shifting,and recombining the phase-shifted frequencies, are all accomplished inthe above-described single integrated device.

Referring back to FIG. 5, the encoded N tributary or channel signals arethen combined prior to being passed through phase scrambler 540 andbeing transmitted over the fiber link and through the network. Thenetwork can comprise a WDM network that allows the signals of the system500 to be transported transparently to the other signals that arenormally carried by the WDM network. In that regard, the system 500advantageously uses a relatively small and tunable window, which iscompatible with WDM systems that are currently deployed. FIG. 8illustratively depicts how an OCDM system in accordance with the variousaspects of the present invention may be overlaid on such a network.Note, however, any other optical network may be used in accordance withthis aspect of the present invention if a tunable source is used. AsFIG. 8 shows, the OCDM signals may be multiplexed into the WDM channel.

After the scrambled signals traverse the network, they are descrambledby descrambler 550 and split and provided to a plurality of matchingdecoders 560 _(1 . . . N). In particular, decoding may be accomplishedby using a matched, complementary code; for the binary codes used here,the code is its own complement and consequently, the coder and decoderare identical. The decoded signal has the pulses restored to theiroriginal position within the bit period and restores the original pulseshape. Decoding using an incorrect decoder results in a temporal patternthat again has zero optical power at the center of the bit period andthe majority of the energy for that pulse is pushed outside the timeinterval where the desired pulse lies.

Implementation of Photonic Layer Security

Referring to FIGS. 5, 9 and 10, implementation of OCDM-based photoniclayer security (PLS) in accordance with an embodiment of the presentinvention is described below.

As stated above, since orthogonal codes are used, the maximum number ofsimultaneous tributaries or channels is equal to the number of frequencybins. For Hadamard codes of order N of (H_(N)) the number of possibleorthogonal codes states so generated is N. An eavesdropper equipped withan adjustable decoder would have to guess only N possible code settingsin order to tune in on any given tributary.

To increase the search space available to an eavesdropper withintentional malicious attacks, the present embodiment includes a phasescrambling methodology.

An orthogonal matrix W_(N) can be generated from H_(N) by premultiplyinga diagonal or monomial matrix D_(N) of order N with all of theon-diagonal elements being arbitrarily chosen phase shifts.

In other words, when random phase settings corresponding to thescrambling code are imposed on all the conventional Hadamard codes, anew set of N distinct orthogonal codes is produced, referred to here asthe modified Hadamard codes. FIG. 9 illustrates the effect of scramblingon four Hadamard-32 signals. Each panel is the simulated temporalintensity variation for two bit periods as might be seen by aneavesdropper. The left panel shows the result of encoding with theoriginal Hadamard-32 codes 6, 7, 9 and 12. The spiky nature of thepatterns and their discrete appearance in the time domain would appearto render the codes vulnerable to detection by an eavesdropper. However,using the corresponding set of scrambled Hadamard-32 results in thesubstantially different time-dependent signal shown in the right panelof FIG. 9. The modified Hadamard-32 has been created by a scramblerusing random 0 and π phase shifts for each element. For this binarychoice of phase setting and diagonal matrix, the search space isincreased from e=32 to ε=2³², assuming all 32 codes are present. Notethat not only has the peak amplitude of the variation been suppressed,but the energy in a bit is now spread throughout the bit period.

Using monomial matrix increases the search space further to ε=32!×2³².More importantly, in addition to shared phase scrambling, the frequencyshuffling decouples the correlation between the attempted guess of thekey unless about 75% of the key has correctly been identified. Thisprocedure ensures that exhaustive search attack is practicallyimpossible. The application of monomial matrix to OCDM-based encryptionis described by G. Di Crescenzo et al. in “On the Security of OCDM-basedEncryption Against Key-Search Attacks,” presented at Summer 2009IEEE/LEOS Summer Topical Meeting on Optical Code Division MultipleAccess, July 2009, Newport Beach, Calif., the contents of which areincorporated by reference in the present application.

It is important to recognize that the random diagonal matrix D_(N) canbe implemented as a separate encoder similar to the same sort used toapply Hadamard codes to the MLL signal. This means one can physicallyseparate the Hadamard coding stages used for directing communicationbetween end users and the diagonal matrix scrambling stages in anetwork. Since it would be desirable to change the scrambling code withsome regularity, the scrambling coders should be dynamically adjustablein synchrony and there is therefore some advantage to sharing theseunits to keep their number small.

As a result, increased capacity has been achieved through inversemultiplexing of the tributaries of a high data rate signal and theimproved security has been created through phase scrambling.

This degree of signal obscuration coupled with the potentially largenumber of possible scrambler states and the ability to dynamicallychange the scrambler code setting at will all contribute to theobscurity of the composite signal. The large code space makeseavesdropping by exhaustive search for the scramble key a practicalimpossibility in a brute force attack.

Implementation of Multi-Channel Interference (MCI) Rejection

Referring to FIGS. 5 and 11, implementation of multi-channelinterference (MCI) rejection in accordance with an aspect of the presentinvention is described below.

MCI noise from undesired tributaries or channels may remain even afterthe signals have passed through the matched OCDM decoder. In addition,since the optical signal energy present in both the decoded andundesired channels are similar in magnitude, both will appearessentially identical from the perspective of a typical photoreceiverthat is band-limited to the data bit rate, preventing the desired signalfrom being recovered correctly. Therefore, further processing techniquesare necessary in order to eliminate the interference.

Given the bandwidth requirements of an OCDM system, which is typicallyon the order of many tens or even hundreds of GHz, the removal ofinterference performed in the electrical domain is impractical due tothe need for ultrafast electronics. As a result, MCI rejection can beperformed in the optical domain. One category of optical processingtechnique for MCI rejection is optical time gating.

Application of optical time gating to extract the decoded OCDM signal isillustrated in FIG. 11. Through the proper selection of an appropriatecode set for a synchronous coherent OCDM system, it is possible todesign the system such that the multi-channel interference energy fallsoutside a time interval where the properly decoded signal pulses reside.Therefore, by optically gating the composite signal in order to providelow loss within the desired time window while at the same time providefor high extinction outside that window, the properly decoded signal bitstream can be extracted.

The described embodiments of the present invention are intended to beillustrative rather than restrictive, and are not intended to representevery embodiment of the present invention. Various modifications andvariations can be made without departing from the spirit or scope of theinvention as set forth in the following claims both literally and inequivalents recognized in law.

1. An optical system for transporting data, comprising a source forgenerating a sequence of optical pulses each optical pulse comprising aplurality of spectral lines uniformly spaced in frequency with fixedabsolute frequency and relative phase; a set of data modulators eachassociated with a tributary/channel and operable to modulate thesequence of pulses using tributary/channel data to produce a modulateddata signal; a set of encoders imparting a plurality of orthogonalcodes, each of the plurality of orthogonal codes being associated withone of the data modulators to spectrally encode the modulated datasignal to produce an encoded data signal; an optical combiner forbit-synchronously combining the encoded data signals into a compositetransport data signal wherein each tributary/channel shares the samespectral bandwidth during transmission; a shared spectral phasescrambler for phase scrambling the composite transport data signal usinga shared scramble code as an encryption key to generate the encryptedsignal; a shared spectral phase descrambler for descrambling theencrypted signal using a shared descramble code as a key to generate adescrambled data signal, the shared descramble code being a complimentof the shared scramble code; and a set of decoders each of which is aconjugate match to one of the encoders, for spectrally decoding aspecified encoded data signal to produce a set of decoded data signals.2. The optical system of claim 1, wherein the source for generating thesequence of optical pulses comprises a mode locked laser wherein each ofthe plurality of spectral lines is approximately equal in amplitude andphase locked.
 3. The optical system of claim 2, wherein the encodercomprises a spectral phase encoder that applies a phase shift defined bya code applied to each of the plurality of spectral lines.
 4. Theoptical system of claim 3, wherein the code is chosen from a set of twoor more mutually orthogonal codes.
 5. The optical system of claim 4,wherein the set of two or more mutually orthogonal codes comprises a setof Hadamard codes.
 6. The optical system of claim 1, wherein the samespectral bandwidth for a group of tributaries/channels is limited to atransparent window in a WDM network.
 7. The optical system of claim 1,further comprising an optical time gate coupled to the output of thematching decoder and operable to temporally extract data signal from thedecoded signals.
 8. The optical system of claim 1, wherein the sharedspectral phase scrambler applies a random phase setting to the pluralityof orthogonal codes, the resulting shared scramble code being used bythe scrambler for scrambling the composite transport data signal.
 9. Theoptical system of claim 1, wherein the set of encoders and the set ofdecoders are comprised of at least one micro-ring resonator.
 10. Anoptical system for transporting data, comprising: a source forgenerating a sequence of optical pulses each optical pulse comprising aplurality of spectral lines uniformly spaced in frequency with fixedabsolute frequency and relative phase; a set of data modulators eachassociated with a tributary/channel and operable to modulate thesequence of pulses using tributary/channel data to produced a modulateddata signal; a set of encoders imparting a plurality of orthogonalcodes, each of the plurality of orthogonal codes being associated withone of the data modulators to spectrally encode the modulated datasignal to produce an encoded data signal; an optical combiner forbit-synchronously combining the encoded data signals into a compositetransport data signal wherein each tributary/channel shares the samespectral bandwidth during transmission; a shared spectral phase andfrequency scrambler for phase and frequency scrambling the compositetransport data signal using a shared scramble code as an encryption keyto generate the encrypted signal; a shared spectral phase and frequencydescrambler using monomial matrix for descrambling the encrypted signalusing a shared descramble code as a key to generate a descrambled datasignal, the shared descramble code being a compliment of the sharedscramble code; and a set of decoders each of which is a conjugate matchto one of the encoders, for spectrally decoding a specified encoded datasignal to produce a set of decoded data signals.